Volume control system for radio receiving apparatus and the like



April 2l, 1936.

F. F. G. HOLST VOLUME CONTROL SYSTEM FO-R RADIO RECEIVING APPARATUS ANDTHE LIKE Filed oct". 21, 4952 @Emil/er with Freoaemycycles per secondPatented Apr. 2l,- 1936 PATENT OFFICE VOLUME CONTROL SYSTEM FOR RADIORECEIVING APPARATUS AND THE LIKE Poul F. G. Holst, Riverton, N. J.,.assignor to Radio Corporation of Delaware America, a corporation ofApplication October 21, 1932, Serial No. 638,838

7 Claims.

The present invention relates to volume control systems for radioreceiving apparatus and the like, and has for one of its objects toprovide an improved system of that character.

It is known that the human ear is less responsive to both high and lowfrequency sounds than it is to sounds in the middle register of `theaudio frequency range when the volume or amplitude of such sounds is ata low level. It is also cornmonly observed that there is usually someoptimal loudness at which certain sounds, such as reproduced music lorspeech from a radio set or phonograph are most pleasing to listen to.The general effect is that music loses its body when reproduced at lowvolumes and speech sounds acoustically tubby when reproduced at highvolumes. The cause for this eifectcan be traced to the variablefrequency response of the ear to tones of different intensity.

When the volume control system of a radio re-` ceiver or similarapparatus such as an electric phonograph is so constructed that it doesnot change the frequency response of the apparatus, and most volumecontrol systems have been built with this aim in view, the reproductionusuallyv sounds most natural when it is reproduced at the same intensityas it would be heard by an observer listening at the place where hewould normally be located to hear the original perform-v Because of aproper regard for the feelance. ings of others, or possibly because ofthe listeners own sensibility, radio apparatus and electric phonographsare very often operated at such a volume adjustment that the soundintensity delivered'by the reproducer is considerably less than that atwhich it would normally be heard. In such cases, the body of the musicis badly lost.

If a volume control system is constructed so that the low frequencyrange of sounds is raised in almost an inverse ratio to the relativesensitivity of the ear in going from normal volume to the volume atwhich the sound is to be heard, a compensation results which makes upfor the different frequency sensitivity of the ear at the lower volume.Such a volume control is known as anacoustically compensated volumecontrol.

Accordingly, it is a further object of the present invention to providean audio frequency volume control system for radio receiving apparatusand the lik-c, adapted to compensate for deficiencies in theresponsiveness of the human ear and to provide predetermined andrelatively high delgrees of compensation throughout certain portions ofthe audio frequency range of operation of (Cl. Z50-20) such apparatus,whereby the effect of the sound output therefrom may be in an apparentimproved quality at low sound levels.

The application of this type of volume control system to a radioreceiver presents certain difficulties and requires the redesign of whathave heretofore been regarded as adequate volume control systems. Theso-called volume control means, in many radio receivers, is not strictlya volume control means, but a sensitivity control 10 means, since itregulates the radio frequency amplication to compensate for thevariation in intensity of the voltage across the antenna and no definiterelation exists between the position of the control and the output levelof the reproduced sound.

It is essential that a deflnite relation bel established betweenreproduced sound output and volume control setting and that this levelbe maintained irrespective of the variation in intensity of the voltageacross the input of the receiver. This requirement necessitates a highgain radio frequency amplifier provided with an automatic sensitivitycontrol capable of maintaining the voltage input to the demodulatorsubstantially constant, irrespective of the variation in signalintensity within the sensitivity limits of the receiver. The fulfillmentof the requirement allows the use of a volume control in the audiochannel of the receiverin which the angular rotation of the controlbears a definite relation to the intensity" of the reproduced sound.With these conditions established, audio compensation can beincorporated in the receiver to give the required aural compensation fordifferent output levels. 35

It is, therefore, a still further object of the present invention toprovide an audio frequency volume control system of the characterreferred to, for compensating, simultaneously with variations in theadjustment of the signal amplitude or volume level, certain portions-ofthe audio frequency range of signals controlled thereby at the lowersound levels and to provide such signals at substantially constantaverage potential.

Broadcast receiver designs in the past have not met these requirementsbecause either no automatic sensitivity control was included in thedesign or the control employed was inadequate to maintain the signal| tothe required level.

In accordance with the invention, therefore, one or more shunt impedancemeans are arranged to be connected gradually across an audio frequencysignal transmission circuit through the medium of a volumecontrolpotentiometer device also connected inshunt with saidtransmission circuit. The arrangement is such that the impedance meansforms a frequency responsive variable load at predetermined tap pointson the potentiometer device, so that the adjustments of volume and ofthe loading are performed simultaneously in a predetermined relation bythe operation of the volume control potentiometer device.

Suitable means is also provided to establish, at the input terminals ofthe potentiometer device, substantially constant average signal inputpotentials. In a radio receiving apparatus, this may be provided throughthe medium-of automatic volume control means. should be of an improvedtype adapted to provide substantially a constant average outputpotential.

By providing a signal of substantially constant amplitude at the inputterminals of the volume control device, it is possible to provide thevolume control network, in which the volume control device may belocated, with a predetermined load, thereby to produce the necessarytone compensation at predetermined volume levels to effect the desiredresponse characteristic definitely and under all conditions ofoperation. Furthermore, as stated hereinbefore, a definite relation maybe established between the setting or adjustment of the volume controldevice and the sound output controlled thereby irrespective of thevariation in intensity of the signal voltage applied to the inputterminals.

By combining, in a radio receiving system, an adequate automatic volumecontrol'system and a compensated audio frequency volume control network,adjustments of volume level may have a predetermined relation to tonecompensation, and the adjustment of volume down to the lower levels ofaudibility may be made without causing any apparent change in the tonequality of the signals or sounds so controlled.

To summarize the foregoing, therefore, it may be briefly stated that, inaccordance with the invention, a volume control network is provided inan audio frequency signal transmission channel of a radio receivingapparatus or the like, and having in connection with the network, meansfor providing input signals therefor of substantially constant averageamplitude. The volume control network includes a tapped potentiometervolume control device connected in shunt with the signal transmissioncircuit and electrically loaded at the tap or load points along itsresistor element, whereby as the volume or Signal level through thechannel is varied by adjustment of said potentiometer device, theloading is automatically applied gradually in shunt with the channel,while the signal input amplitude is held substantially constant. Theelectrical loading means includes circuit impedance elements arranged tohave a certain predetermined frequency response characteristic operativeto provide a frequency variable shunt path or paths for portions of thepotentiometer resistance.

With constant input potential from the signal source to the audiofrequency signal transmission channel or volume control network, it willbe seen that the loading of the circuit may be made to have apredetermined relation yto the volume level adjustment of the volumecontrol device, thereby toprovide a desired compensation for that level.By providing such compensation and loading at various points along thevolume control device, a predetermined compensation may be appliedthroughout the range of operation of said device, whereby a certainportion or portions of the audio frequency signal However, such meansrange may be affected to cause signals in said range or ranges to beattenuated or compensated to provide tone correction in the lower soundor signal output levels of operation.

The practical advantages of radio receiving` apparatus having automatictone compensation with volume control in conjunction with automaticvolume control means, are readily appreciated when compared with theeffect upon the quality of a received signal as the signal output of anordinary radio receiver is reduced to the lower limit of what may betermedfcomfortable room volume by the usual volume control means. Itwill be noted that the tone of the reproduced signal from the ordinaryreceiver is apparently reduced in frequency range and seems to lack boththe low and the high frequency portions of the audio frequency range. Asthe signal level is further reduced in volume, the bass and the trebleportions of the musical range seem to disappear, leaving only the middleregister in the signal sound range.

The invention, however, will be better understood from the followingdescription when taken in connection with the accompanying drawing,

and its scope will be pointed out in the appended claims.

In the drawing, Figure l is a simple wiring diagram of a portion of anelectrical signal transmission circuit provided with a volume controlsystem embodying the invention.

Fig. 2 is a curve diagram illustrating a feature of the operation of thevolume control system shown in Fig. 1;

Fig. 3 is a circuit diagram of a radio receiving apparatus provided witha volume control system embodying the invention; and

Fig. 4 is a curve diagram illustrating an operatingv characteristic of apreferred form of a volume control means employed in the circuit of Fig.3.

Referring to Fig. 1, 5-5 are the input terminals, and 6-6 are the outputterminals of a signal transmission circuit 1, representing any audiofrequency signal transmission channel or circuit to the input terminalsof which substantially constant average signal potentials are applied.Connected in shunt to the circuit, for the purpose of controlling theamplitude of the signals transmitted through said circuit and applied tothe output terminals 6, is a volume control potentiometer device 8having its resistor end terminals connected with the input terminals 5,and a movable arm or contact 9 connected, together with one of its endterminals, with the output terminals 6, thereby to provide aconventional volume control system for a circuit of that character.

Referring to Fig. 2, this volume control system operates in accordanceWith curve I0, plotted with reference yto frequency in cycles persecond, and signal transmission efliciency E, with varying frequency.This curve indicates that a device of this type will have asubstantially straight frequency response characteristic and,accordingly.;`

for any adjustment of the volume control 8, by adjustment of the tap 9,will be substantially unaffected by variations in frequency of signalstransmitted through the volume control network. While such apparatus, sooperated, delivers the signal output without actual change in fidelity,the effect lupon the listening ear is that the fidelity has beennoticeably changed.

In order to compensate for the deficiencies in the responsecharacteristic of the human ear. and

' to provide a predetermined required sound intensity effect thereon,which is substantially constant throughout the audio frequency range atthe lower sound levels, itis necessary to provide a volume controlsystem which, at the lower volume control adjustments for low levelsound output, will have a fldelity or frequency response curve,indicative of its transmission efficiency at various frequencies of thenature of the curve indicated at II in Fig. 2. From an inspection ofthis curve, it w'lll be seen that a volume control system forcompensating for the lack of responsiveness of the. human ear to higherand lower frequency sounds, provides for accentuating said sound rangesand attenuating the middle register, such as between 500 and 2000cycles, to which the ear is more effectively responsive.

It will be appreciated that, in order to produce an audio frequencyoperating characteristic of the character above described, asubstantially constant signal is required upon the volume controlnetwork. The latter is of the shunt type variably loaded to graduallybring into operation the shunt frequency variable impedance elements,all in a definite relation to the volume control adjustment, withoutwhich the system would be substantially ineffective. To this end, thepotentiometer 8 is provided with at least one tap point I2 a.'ong itslength, and preferably more adjacent to the low potential end, and tothis tap is connected a loading impedance Z, indicated at I3, having acertain predetermined variable frequency response characteristic and acertain predetermined relative impedance value within thesignalfrequency range, in which signal attentuation is desired. 'I'hisimpedance value must be lower than and must bear a certain predeterminedrelation to the impedance of the low potential end or portion of thepotentiometer device to which it is, through the tap point and the lowpotential end, connected in parallel.

The loading impedance, indicated at I3, may include a resistancecapacitance network and various circuit impedance elements so arrangedand related, as hereinafter described, that the impedance means may havea very low impedance value at the frequency, or within the frequencyrange, at which minimum response is desired, such as, for example,approximately 1500 cycles, or between 500 and 2000 cycles. 'Ihis valueis with respect to the impedance of the low potential end of thepotentiometer device between the tap point and the low potentialterminal, that is, designating the high potential terminal of thepotentiometer device as I4 and the low potential terminal as I5, thenthat portion between the terminals I2 and I5.

In addition, the impedance means indicated at I3 preferably should haveaxrelatively broad tuning response characteristic 'at the frequency atwhich minimum response is desired. With an arrangement of thatcharacter, the transmission response characteristic of the circuit andvolume control means may have a certain low or room volume adjustmentsuch as at the tap point I2, by moving the slidable arm or contact 9thereto, and be made to correspond to and have an operatingcharacteristic which may be represented by the curve I I.

Furthermore, with a compensated volume control system of the abovecharacter. advantage 1 also may be taken of the fact that, at low volumecontrol adjustments of the device 8, the output terminals 6 areconnected by an impedance which becomes gradually lower as the volumecontrol setting is reduced, thereby permitting succeeding devices orapparatus (not shown), to change their or its frequency responsecharacteristic. For example, it will be appreciated, as apparent to oneskilled in the art, that the leakage peak in an audio frequency couplingtransformer, if connected to the terminals 6, may be caused to increaseappreciably, thereby causing the frequency response characteristic, inconjunction with changes in the adjustment of the volume control device8, to be accentuated'and multiplied.

It should be noted that the curve II in Fig. 2 is that provided when themovable contact 9 is substantially at the tap point I2, while the curveI0 indicates the response characteristic when the contact 9 is at thehigh potential end I4 of the volume control device. The responsecharacteristic of the system for other adjustments of the contact 9 willbe located between and conform to curves I0 and II, approaching in shapemore nearly the one or the other of said curves to which they are moreadjacent.

The volume control potentiometer device 8 should be, and preferably is,tapered in resistance from the high potential end It to the lowpotential end I5 to produce Variations in signal amplitude of volume, ina suitable manner, preferably being tapered in accordance with alogarithmic curve, thereby to produce equal changes in sound effect uponthe ear for equal changes in ladjustment of the volume control device. Apreferred form of response curve for a volume control device embodyingthe invention will be hereinafter described.

An additional advantage of a compensated volume control system of thecharacter described, is that the range of operation includes a zerosetting and is obtained by reason of the fact that the loading means forthe circuit is so connected therewith, that it is not interposed inseries between the controlled circuit and the terminals of the volumecontrol device. In the present example, it will be noted that the volumecontrol tap 9 may be moved between the high potential end I4 and the lowpotential end I5, thereby to change the volume adjustment from maximumto full zero. In a radio receiver, for example, this has the distinctadvantage that the output of the receiver may be cut dead at such timesas such adjustment is desired.

It`will be noted that other tap points are provided on the resistorelement 8 at I6 and I1, to which additional impedance elements may beconnected in the same manner as impedance element I3 is connected to thetap point I2 and for the same purpose. It has been found, however, thatthe circuit employing a single tap point is satisfactory.

The combination of a shunt volume control potentiometer device withshunt impedance devices or means connected with tap points on thepotentiometer device and having a constant average signal inputpotential applied to the Acircuit has the advantage that not only maythe tap point or points be gradually connected elfectively to thecircuit, but the circuit involves no series impedance elements throughwhich all components of the signal must pass. It is provided only withshunt impedance means each of which pass only such signal energy as itis designed to compensate or attenuate. within other frequency rangesare less affected by the effective impedance of the compensating uorattenuating means.

Referring now to Fig. 3, I8 indicates a radio Signals f 4 receiverprovided with an automatic volume con.. trol system and having inputterminals I9 connected with suitable energy collecting means, such as anantenna 20 and ground connection 2|, and having output terminals 22connected with a audio frequency signal channel 23 through the medium ofan audio frequency amplifying or detecting tube 24.

The signal potential applied to the tube 24 and, therefore, to the audiofrequency signal channel 23 from terminals 22, may be indicated at E2and is substantially constant in response to a variable signal potentialE1, applied from the collecting means to the input terminals I9.

Any suitable receiving apparatus may be employed and any suitableautomatic volume control system may be provided in conjunction therewithto deliver tothe input end of the audio frequency signal channelsubstantially a constant average signal potential. With an arrangementof that character, the audio frequency tube-24, which may be the audiofrequency detector, is caused to receive a constant signal potentialwhereby the volume control means for the apparatus may be placed whollyin the audio frequency channel.

In the present example, the tube 24 may be considered as a detector tubeand the audio frequency signal channel 23 is included in its anodecircuit. Accordingly, the one side of the signal channel is connectedwith a source of anode potential (not shown) through an audio frequencycoupling impedance or choke coil 25. The

circuit is also provided with the usual radio frequency filter circuitindicated by the filter` devices 26.

Connected in shunt with the audio frequency transmission channel 23 is avolume control potentiometer device 21 having input terminals 28 and avari-able contact element 29. A tap for a shunt impedance load isprovided at 30, more adjacent to the low potential end of thepotentiometer device, and is connected with the low potentialv end ofsaid device through a series choke coil or inductive reactance 3| and acondenser or capacitive reactance 32, forming a tuned circuit in shunt`with the low potential end of the potentiometer device. The variablecontact member 29 and one terminal 28 of the potentiometer device areconnected with the primary 33 of an output transformer 34 through a tonecontrol network 35 comprising a series choke coll 36, a shunt condenser31, and a potentiomf eter rdevice 38 interconnected in such a mannerthat the choke coil may be connected in series with the transmissioncircuit and the condenser in shunt therewith, or both maybe effectivelyremoved from the circuit gradually by operation of the potentio-meterdevice.

This tone control network per se, forms no part of the present inventionand, therefore, is believed to require no further description. Forexample, it may be of the type shown in Fig. 2

of Bierwirth Patent, 1,997,762, and described and claimed therein! Itwill be seen that the volume control potentiometer device 21, being ofthe shunt variable contact type, is adapted to provide a variable outputimpedance with respect to the primary 33 of the output transformer 34.By this means, a variation in the performance of the tuning between theleakage impedance and the shunt stray capacity of the secondary 39 maybe provided to assist in the compensating action of the volume controldevice.

satisfactory in a circuit of the character shown,

comprising an output circuit for an audio frequency tube in connectionwith the primary of an audio output transformer. The tap point 30 in asystem of that character is preferably placed substantially at 6000 ohmsabove the low potential end of the potentiometer device and theimpedance elements 3| and 32 may have values from 50 to 80 millihenriesand .5 mfd. respectively, with a total ohmic resistance in the chokecoil 3| of substantially 520 ohms.

It will be seen that, with the tap 29 of the potentiometer device at theload point 30, an ohmic impedance of 520 ohms is then placed in shuntwith 6000 ohms. The condenser 32 Iand the choke coil 3| tune broadly tofrequencies in a signal range to be attenuated, such as the r-angeincluding 1500 cycles, whereby the operating response may correspond tothe desired output curve indicated at in Fig. 2. Since the condenser andchoke coil are tuned to such a range of frequencies, it will be seenthat signals in said range will be highly attenuated because of the loWimpedance, (520 ohms), of the shunt path from the tap or load point 30tothe low potential side of the circuit.

More specifically, the impedance ratios provided in the'v'olume controlnetwork of the present example are such that the middle register of theaudio frequency range to which the ear is more sensitive, are attenuatedapproximately 48 times with respect to the higher and lower frequencyportions of the audio frequency range. This will be seen from aconsideration of the ratio of the selected impedances when the variablecontact 29 is located, -for maximum attenuation, opposite the'fixed tapconnection 30, in which position the ratio of impedances areapproximatelyl 520 to 24,520, or 1:48, for the middle register of theaudio frequency range to which the impedance means 3|-32 is responsive.This is disregarding the shunt impedance of 6000 ohms across theimpedance means 3 |-32.

Within the signal range to be attenuated, substantially the onlyimpedance in circuit between the tap point 30 and lower end of thevolume control network is the ohmic resistance of the inductance 3| andthe higher resistance of the lower section of the volume control device.The inductance is arranged to have approximately denser 32 are lesseffective, and the impedance ratios are substantially those provided bythe two sections of the potentiometer device in the ratio of 6000 to24,000 or substantially 1 to 5. Therefore, the signals outside the rangeof signals to be attenuated are reduced only five times as com.

pared with substantially 48 times for the range above described.

yf'lhe ratio o f the impedance values provided by the potentiometerdevice and the shunt impedance network may be considered in the circuitin which it is employed, by assuming a certain output impedance for thetube or device to which it is coupled.

For example, referring to Fig. 3, the output impedance of the tube 24may be considered as 20,000 ohms, then with a resistance of 30,000 ohmstotal for the potentiometer device 21, and a resistance of 6,000 ohmsbetween the tap point 30 and the low potential terminal 28, an impedanceof 520 ohms for the network 3I--32 has been found to provide a desiredattenuation of signals within the range to which the network is tuned,such as between 500 and 2000 cycles. Considering the output voltagedelivered to the output circuit connected with the potentiometer contact29 when the latter is at the point ofmaximum compensation 30, saidoutput voltage will be equal to4 approximately. The compensationprovided by the system at the tap point 30 may thus be expressed as orsubstantially 12. subtracting the initial compensation of 1, thisprovides a compensation of substantially 1100%.

This maximum compensation may be compared with the compensation atanother point, for example at a point mid-way between the high potentialterminal 28 and the point of maximum compensation 30, by the sameprocedure. For example, the compensation for the frequency rangel to beattenuated may be represented by the im' pedance ratio of Vwhile thecompensation outside this range at the high and low frequency ends ofthe audio frequency range maybe represented by the fraction Comparingthese two fractions, the maximum given, but that the compensationA isrelatively high at the maximum point, as is also desirable and'necessary to obtain the proper effect upon the ear at low sound outputlevels.

Inhconsidering compensation for acoustical effects in a. volume controlsystem of the present invention, it is necessary to determine at whatsound level, or wattage output the maximum compensatlon should beemployed to produce the desired effect upon the ear, for the reasonthat, as has been hereinbefore explained, the ear is less sensitive tosounds in the high and low frequency ranges at low sound levels thanitis at the higher levels above what may be termed normal room volume.

. Accordingly, in providing a. volume control system embodying theinvention, the volume control device is so designed that the tapcompensating points may be provided at positions to which the device isadjusted for sound levels at which compensation is most effective. Forexample, if a maximum output of 20 watts is available, the maximumcompensation for tone or acoustic effects is arranged to be provided ata. sound output of substantially one watt or less in a radio receivingapparatus, although other ratios for radio receivers and other apparatusare necessary to meet differing operating conditions.

From the foregoing description, it will be seen that by changing theconstants of the circuit elements 3l and 32 and by changing the tappoints or increasing the number of tap points as in the circuit shown inFig. 1, other operating characteristics may be given to the volumecontrol action.

In itself, the volume control action is preferably logarithmic, wherebythe ear may perceive equal increases in volume level for an equal degreeof movement of the control .element of the volume control device. It hasbeen found in a volume control system that the action of the volumecontrol device is preferably logarithmic above and below each tap pointand, in accordance with the invention, a volume control potentiometerdevice, having an operating characteristic represented by the curve 40shown in Fig. 4, is preferred. Referring to Fig. 4, this curve isplotted between resistance in ohms and rotation of the control elementof the potentiometer device. It will be noted that the curve islogarithmic in shape, between the zero resistance point indicated at 4|and a point 42 representing the tap point 30 of the device shown in Fig.3. It is also logarithmic between point 42 and a point 43, or maximumresistance point. Resistance values' between the various terminals areindicated on either side of the curve for a better ,understanding of theoperation of the potentiometer device.

.contact of the potentiometer device reaches the loading point whereatmaximum compensation is obtained, the impedance from the movable contactto the low potential side of the audio frequency transmission circuit isno longer determined solely by the portion of the potentiometer resistorbetween said tap and the low potential side of the circuit. It is thendetermined also by the loading connected with the tap point, andadjustments of the volume control device below the tap point take from apotential existing across terminals 30 and low potential terminal 28, as

-ing equal increment tapped portion determined by theloading.Accordingly, it has been found desirable and eifective'in operation, toprovide a second logarithmic taper from the tap point to the lowpotential side of the potentiometer device for operation in accordancewith the curve shown in Fig. 4, when a single tap point is provided.

As is well known and understood, by logarithmic taper is meant thatchanges in resistance, for example, between the movable tap 29 and aterminal of the potentiometer device, such as the high potentialterminal 28, as the contact 29 is moved away from said terminal alongthe resistance element of the device rtoward the opposite terminal, isat first rapid and then gradually less for equal distances moved alongthe resistance element. The decrease in change of resistance for equaldistances vary in accordance with a logarithmic curve and for thepurpose of producing a signal output effect of a correspondor decrementcharacter.

In the circuit described above, the attenuation of voltage, with the arm29 at the tap 30, is -14DB with the compensation circuit 3|-32disconnected. Connecting the compensation circuit results in anadditional attenuation of -34DB for frequencies in the region of 1000cycles (500-2000 cycles). The voltage output at cycles is up 16DB fromthat at 1000 cycles at the point of maximum compensation.

Reducing the volume control setting below the point of maximumcompensation has little effect on the shape of the voltage frequencycharacteristic of the volume control circuit. Care must be exercised indetermining the value of the of the volume control in orderthatsubsequent circuit constants are not effected when the volumecontrol is adjusted to positions below the point of maximumcompensation.

Referring. again to Fig. 3, mea'ns are provided between the input end ofthe audio frequency transmission channel and the volume control device21 for controlling the transmission efficiency of the circuit to reducethe effect of hiss and high frequency noises when receiving underadverse conditions.

This means includes two condensers 44 and 45 arranged to be selectivelyconnected in series with one side of the transmission channel by aselector switch 46. The condensers have widely different impedancevalues, the condenser 44, for example, having a value suitable forattenuating certain portions of the high frequency end of 'the range,and the condenser 45 being adapted to transmit freely all signals withinthe audio frequency range for greater fidelity of operation.

From the foregoing description, it will be seen that a volume controlsystem, in accordance with the invention, provides for loading an audiofrequency transmission circuit by shunt signal attenuating means,gradually bringing such attenuating means into operation successively toload the circuit within predetermined frequency ranges of operation atlower levels of audibility and that this control is effected by simplemeans in a circuit supplied by signals of constant average amplitude andamplified thereby to produce'a response characteristic which, regardlessof variations in input signal potentials and similar operatingconditions of the apparatus, may

produce a sound intensity effect upon the ear, of an adjustment ofvolume level only, without apparent change in fidelity or tone quality.

The problem of using the acousticaliy compensated volume control with aphonograph is much simpler. Most records are recorded with the averageintensity maintained close to some standard level and any adjustment ofthe frequency compensation for one record holds'for practically allother records. 'I'he volume control may, therefore, be compensatedwithout difilculty as the input signal is automatically controlled involume by the record.

It will be understood that various modificationsf within the conceptionof those skilled in the art are possible without departing from thespirit of my invention or the scope of the claims.

I claim as my invention:

1. In an audio frequency volume control system, a tapped volume controlpotentiometer device comprising a resistor element having at least tworesistor sections, means providing a fixed tap connection between saidsections and a variable contact movable along and in connection withsaid element, frequency discriminating impedance means connected inshunt with one of said sections through said tap connection, said meanscomprising a single inductor and a single capacitor only in series, andan output circuit therefor connected with Vsaid variable contact.

2. In a radio receiving apparatus, the combination of a detector havingsubstantially a constant audio frequency signal output, an audiofrequenc signal transmission circuit following said detector fortransmitting audio frequency signals therefrom, a volume control networkin said circuit and including a potentiometer volume control devicehaving a resistor element comprising two differing resistance sectionsand a tap connection between said sections, one section being less thanone half the resistance of the other section, an impedance meansconnected with said tap connection across the lower resistance sectionof said resistor element, said impedance means including a seriesconnected inductance and capacity tuned to a predetermined range ofsignal frequencies and having within said range of frequencies animpedance relatively lower than the impedance of the resistance elementsection across which itis connected, and a variable contact element forsaid potentiometer device for controlling the load applied to saidcircuit by said impedance means.

3. In a radio receiving system, automatic volume control means forproviding substantially constant average signal amplitude, an audiofrequency signal transmission circuit for receiving signals controlledby said automatic volume control means, and audio frequency volumecontrol means including a tapped resistor in shunt with said circuit, anoutput circuit having a variable tap connection with the resistorsubstantially throughout its length for'volume control, a re- `actancenetwork connected in shunt with a portion of said resistor including thelow volume end thereof, said portion including less than one third ofthe total resistance of said resistor, said network and said portion ofsaid resistor having a predetermined impedance ratio whereby tonecompensation is provided simultaneously with volume control, thecompensation increasing as the volume control tap connection is moved toa position to reduce the volume.

4. In a radio receivingl apparatus, the combination of an audiofrequency signal transmission circuit, a volume control network in saidcircuit including a potentiometer volume control device having aresistor element comprising two differing resistance sections and a tapconnection between said sections, one section having a resistance atleast lower than one half the resistance of the other section, animpedance means connected with said tap connection across the lowerresistance section of said resistor element, said impedance meansincluding a series connected inductance and capacity tuned to apredetermined range of signal frequencies and having within said rangeof frequencies an impedance relatively lower than the impedance of theresistance element section across which it is connected, and a variablecontact element for said potentiometer device for controlling the loadapplied to said circuit by said impedance means.

5. In an audio frequency volume control system, a control deviceincluding a shunt variable resistor element, a tone compensatingreactance means connected in shunt to a fixed portion of said elementless than the whole, said means being tuned to provide a minimumimpedance to signals within a predetermined audio frequency range,wherein attenuation of said signals is desired, and variable tonecontrol means connected to additionally control signals transmittedthrough said control device independently thereof in a differingfrequency range.

6. In an audio frequency volume control system, a control deviceincluding a shunt variable resistor element, a tone compensatingreactance means connected in shunt to a xed portion of said element lessthan the whole, said means being tuned to provide a minimum impedance tosignals Within a predetermined audio frequency range, whereinattenuation of said signals is desired, and a second control deviceassociated in circuit with said first device for independentlycontrolling the frequency characteristic of signals in a differing audiofrequency range.

'7. In an audio frequency signal transmission network, volume controlmeans having fixed compensation for one frequency range and variabletone control means connected therewith responsive to signals in adiffering frequency range.

POUL F. G. HOIST.

